Abstract: When designing casks for the shipment of Spent Nuclear Fuel (SNF), one of the most important considerations is the external dimensions of the fuel assemblies to be shipped. Since SNF assemblies tend to grow and deform during irradiation, the post-irradiation dimensions are rarely the same as the pre-irradiation dimensions which are commonly available. In preparation for the issuance of a request for proposals for contingency cask designs by the Civilian Radioactive Waste Management System (CRWMS) Maintenance and Operations (M&O) contractor, the factors which cause SNF assemblies to deviate from their pre-irradiation dimensions were examined. The results of the examination indicated that envelopes of 178.8 inches (454.2 cm) long by 5.6 inches (14.2 cm) square for Boiling Water Reactors (BWR) and 194.8 inches (494.8 cm) long, with control rods, by 8.5 inches (21.6 cm) square for Pressurized Water Reactors (PWR) should accommodate 90% of all assemblies currently in inventory.